In known apparatus and processes for gas phase coating of workpieces, for example, turbine blades, the workpieces are arranged, for example, in the upper region of a container. In its lower region, the container comprises a carrier gas inlet and a carrying grid for the acceptance of a donor metal granulate and an activator powder. The container is annealed for a certain time under argon or H.sub.2 and a donor metal gas forms in the container in the proximity of the donor metal granulate.
The problem thereby arises that the heavy donor metal gasses that form must rise from the donor metal granulate to the parts to be coated, whereby a dilution of the donor metal gasses occurs in vertical direction according to the barometric altitude formula, thereby leading to different coat thicknesses on the parts dependent on their geodetic height relative to the donor metal. Particularly given containers with large payload spaces, a pronounced coat gradient arises in barometric dependence from bottom to top due to the heaviness of the coating gas. It is thereby of no consequence whether the donor metal is distributed at the floor of the container and/or under the cover of the container or, respectively, reactor or whether a molten donor metal is employed. When coating inside surfaces of hollow parts, the additional problem arises that the coating thickness decreases the farther the surface to be coated lies in the inside of the part.
Attempts have been made to control the gas quality and to improve it to a certain extent with a gas rinse conduit conducted into the reactor. This arrangement, however, only functions with adequately slice coat gradients given payload spaces having a low height. A disadvantageous coat gradient particularly arises given payload space heights of approximately 120 mm and more, i.e. the coated part has different coat thicknesses dependent on its respective height in the reactor during the coating.
WO 92/08821 discloses an apparatus for gas diffusion coating of hollow workpieces with a container that comprises a gas admission and a gas discharge, whereby the gas discharge follows the inside surfaces of the workpiece to be coated. In this method, the workpieces are held at a geodetically low height relative to the donor metal, whereby the donor metal is present in the form of a donor metal body that, while preserving a gap, completely envelopes the outside surfaces of the workpiece to be coated. A carrier gas conducted into the container from above is conducted past the donor metal body arrangement that is arranged above the part to be coated and around the part. Subsequently, the coating gas is conducted through the inside of the part and discharged at the underside of the container. In order to assure a high donor metal concentration in the area of the part, the gas discharge is formed as overflow or, respectively, siphon, so that a donor metal gas sump forms in the area of the part. However, this known apparatus requires an involved arrangement and fashioning of the donor metal body that, over and above this, must be exactly aligned relative to the part and must be matched to its shape. A coating of a variety of parts with different geometries requires respectively different donor metal bodies, which is involved and cost-intensive and leads to low flexibility. Over and above this, a barometric dependency of the donor metal gas concentration also exists given a siphon arrangement.
Therefore, there is a need for a gas phase coating process and an apparatus for gas phase coating which provides a more uniform coating of outside and inside surfaces of the part throughout the entire payload space of the reactor given low outlay and high flexibility.